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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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GLACIATION OF THE YAEUTAT BAY REGION                211
greater series of glaciers. When a piedmont bulb is formed, the ice spreads as long as the supply is maintained, and, as ablation proceeds, the surface of the bulb becomes coated with moraine, if the supply of incorporated d6bris is sufficient. Under such conditions an interior flat should not be expected. But if a period of stagnation is succeeded by readvance, and especially if the advance be rapid, the spreading of the ice is interfered with by the stagnant outer part of the piedmont bulb, and, at the appropriate place, rising of lower ice occurs, introducing an area of clear ice where none existed previously. According to the thickness of the outer glacier, its degree of stagnation, and the rate and duration of the advance, the position and extent of the interior flat will vary. Thus with two advances of different intensity, or with different ice conditions, two interior flat areas, in different portions, may develop in the same glacier, as has occurred in the Variegated Glacier.
Marginal Deposits. Tue deposits accumulating around the margins of the Alaskan piedmont glaciers are complex and interesting. The most notable fact is that they are, in the main, water-laid. Doubtless by the melting of the ice a veneer of ground moraine is being accumulated on the site of the glaciers; doubtless, also, upon this is accumulated a veneer of angular d6bris from the ablation moraine; and doubtless there are subglacial deposits of water-laid material, such as eskers; but in the areas occupied by the piedmont bulbs these deposits are for the most part masked by deposits from the water which issues from the wasting glaciers. The burial of these deposits is greatly aided by the slowness of wasting of the moraine-covered glaciers, as a result of which even the glacier ends themselves are at times buried beneath extensive alluvial deposits. Since there is no persistent advance of the piedmont bulbs there is no opportunity for the development of fringing moraines. The glaciers attain a position during advance and spreading, and they retain this position for a long time, not through continual supply, but because of the protection from ablation which their moraine cover supplies. Abundant water issues during the long period of wastage, and it bears enormous loads of sediment. Hence water deposits assume immense importance. Probably far the greater part is deposited in the neighboring sea, to which nearly all of the clay is carried; but the greater portion of the bowlders, pebbles, and sand accumulate either in alluvial fans fringing the glaciers, in deltas on the fiord shores, or in the beaches between the deltas.
If one of these glaciers should completely disappear, there would be left, as the most conspicuous deposit, a broad, crescentic frontal zone of coalescing alluvial fans, very coarse in texture near the glacier fronts, and grading to gravel and sand away from it, with clay beyond on the fiord bottom. On the inner margin, in favorable places where the gravels rested on ice, a hummocky deposit would develop as a result of irregular settling during the melting of the buried ice, with pond and small lake areas within it. Thus a morainic topography, kame-moraine would perhaps be betterówould develop in a more or less perfectly crescentic area around the outer margin of the piedmont bulb, grading outward into the more even slopes of alluvial fans and outwash gravel plains. On the inner side of the morainic crescent would be a depression, with irregular surface and with a veneer of coarse angular fragments. The site of the depression is often occupied by a lake.
Leading from the crescentic moraine, on either end of the crescent, there would extend a marginal band of deposit, also water-laid, marking the sites of the marginal streams, and extending well up into the mountain valleys. These marginal stream deposits